Fish meal is a product obtained by drying and grinding fish or fish waste to which no other matter has been added. The fish or fish waste is usually cooked, the oil and water removed by pressing, and the solids dried to approximately 10% water content. The oil content can vary from 5 to 20%, depending on the raw material and the processing.
Owing to the high unsaturation of fish oil, fish meal is readily autoxidised, with a consequent decrease in nutritional and energy values. Lipid autoxidation in fish meal also causes heat-production which can even lead to spontaneous combustion of stored meal. If a low quality raw material is used, oxidation is likely to be faster.
The risk of spontaneous heating in fish meal can be decreased by "curing", i.e. heating the meal and allowing the autoxidation to proceed before storing or transport. However, if the nutritional or energy levels are to be maintained, anti-oxidants must be used.
Ethoxyquin has been widely approved for us in animal feed and is the most commonly used anti-oxidant in fish meal intended for that use. Despite its widespread use as an anti-oxidant in fish meal, the mode of action of ethoxyquin is unclear. For example, while the effect of adding the compound to fish meal may be observed for a period of say, two years, analysis of the composition fails to detect the presence of ethoxyquin per se within a matter of hours.
Ethoxyquin is a compound of formula I (see below) in which R=Et, X=H. Its preparation and its use as a rubber anti-oxidant, are described in GB-A-No. 0505113. Compounds of formula I (R=C.sub.1-5 alkyl and X=H), including ethoxyquin, and their use as herbicides, are described in U.S. Pat. No. 2661277.
Bharucha et al, J. Agric. Food Chem. 33(5) (1985) 834-9, disclose that ethoxyquin is a potent inhibitor of nitrosamine formation in bacon, and also that 8-nitrosoethoxyquin (I:R=Et, X=NO) and 8-nitroethoxyquin (I:R=Et, X=NO.sub.2) are apparently formed in situ.
Other known 8-substituted ethoxyquin analogues are 2,2,4,8-tetramethyl-6-methoxy-1,2-dihydroquinoline (I:R=X=Me) and 8-hydroxyethoxyquin (I:R=Et, X=OH). The former is disclosed as a pressure-sensitive dye precursor in Chem. Abs. 95 (1981) 99338n, and the latter as a metabolite of ethoxyquin, in the rat, by Skaare, Xenobiotica 9(11) (1979) 659-668.